1. Field of the Invention
The invention relates to an electronic integrator circuit, and more particularly to a fully differential switched-capacitor integrator for use in low noise filters and sigma delta modulators.
2. Description of the Related Art
Fully differential circuits are more attractive in high performance, high precision applications for their two advantages
a) they are less sensitive to power supply noise and PA1 b) they are capable of achieving a dynamic range which is approximately twice that of single-ended circuits. However, fully differential circuits usually just accept well balanced differential input signals. Therefore, a single-ended to differential converter circuit is needed in front of it. This increases the complexity of the system and adds extra noise as well.
There are few related art circuits which convert a single-ended signal to a differential signal by themselves without an extra converter circuit.
U.S. Pat. No. 4,647,865 (Westwick) proposes a switched capacitor input structure for a fully differential amplifier, which can accept single-ended signals. But the offset and low frequency noise of the amplifier do not cancel. This is not acceptable in some high performance applications.
U.S. Pat. No. 5,410,270 (Rybicki et al.) a fully differential amplifier having offset cancellation is proposed. However, this circuit can only be used as an amplifier, it cannot change to an integrator, which is widely used in filter circuits and other applications.
U.S. Pat. No. 4,896,156 (Garverick et al.) proposes a three phase system which complicates the timing and system design.
U.S. Pat. No. 5,220,286 (Nadeem) discloses a single-ended to differential input converter which is more complex than the circuit of the invention and cannot compensate the offset and finite operational amplifier gain either.
U.S. Pat. No. 4,746,871 (de la Plaza) describes a differential switched capacitor integrator but which requires two operational amplifiers and does not appear to accept single-ended inputs.
It should be noted that none of the above-cited examples of the related art provide a fully differential switched-capacitor integrator that could accept single-ended or unbalanced input signals and compensates the offset and finite gain of the operational amplifier at the same time.